Method and system for measuring geometric parameters of through holes

Abstract

A method of measuring geometric parameters of through holes in a thin substrate includes acquiring images of select sub-volumes of the substrate using an optical system having a field of depth greater than a thickness of the substrate. The acquired images are processed to determine the desired geometric parameters.

The invention claimed is: 
     
       1. A method of measuring geometric parameters of through holes in a substrate, comprising:
 positioning a camera comprising an image sensor and a lens having a depth of field greater than a thickness of the substrate at a measuring station, the image sensor having a pixel count of at least 10 megapixels and a pixel size of 1.7 microns or less; 
 positioning a select sub-volume of the substrate within a field of view of the camera and within the depth of field,
 the select sub-volume having a front side, a back side, and at least one through hole, and 
 the at least one through hole having a first side at the front side of the select sub-volume and a second at the back side of the select sub-volume; 
 
 illuminating the select sub-volume with a light source; 
 capturing an image of the select sub-volume on the image sensor; and 
 processing the image to determine at least a minimum transverse dimension of the at least one through hole along a length of the through hole, the minimum transverse dimension of the through hole being between the first side and the second side of the through hole, 
 wherein the light source and the camera are arranged on a same side of the substrate. 
 
     
     
       2. The method of  claim 1 , wherein illuminating the select sub-volume with the light source further comprises reflecting light from the light source into the select sub-volume using a mirror. 
     
     
       3. The method of  claim 2 , further comprising moving the mirror with the substrate. 
     
     
       4. The method of  claim 2 , wherein the light source and the camera are disposed on the front side of the select sub-volume and the mirror is disposed on the on a back side of the select sub-volume. 
     
     
       5. The method of  claim 1 , wherein the light source is a coaxial light source. 
     
     
       6. The method of  claim 1 , wherein the substrate has a thickness of 1 mm or less. 
     
     
       7. The method of  claim 1 , wherein the lens is a telecentric lens. 
     
     
       8. The method of  claim 1 , further comprising effecting a lateral relative motion between the camera and the substrate to position another sub-volume of the substrate within the field of view of the camera and within the depth of field. 
     
     
       9. The method of  claim 8 , further comprising illuminating the another sub-volume with light, capturing an image of the another sub-volume on the image sensor, and processing the image of the another sub-volume to determine at least one geometric parameter of at least one hole in the another sub-volume. 
     
     
       10. The method of  claim 9 , wherein illuminating the another sub-volume comprises effecting a lateral relative motion between the light source and the substrate such that the another sub-volume is within an illumination volume of the light. 
     
     
       11. The method of  claim 1 , wherein processing the image comprises detecting changes in contrast in the image. 
     
     
       12. A system for measuring geometric parameters of through holes in a substrate, comprising:
 a camera comprising an image sensor and a lens having a depth of field greater than a thickness of the substrate, the image sensor having a pixel count of at least 10 megapixels and a pixel size of 1.7 microns or less, and the camera positioned at a select working distance relative to the substrate such that a select sub-volume of the substrate is within a field of view of the camera and within the depth of field,
 the select-sub volume having a front side, a back side, and at least one through hole, and 
 the at least one through hole having a first side at the front side of the select sub-volume and a second at the back side of the select sub-volume; 
 
 a light source for illuminating the select sub-volume; and 
 a processor configured to receive image data from the camera and determine from the image data at least a minimum transverse dimension of the least one through hole along a length of the through hole, the minimum transverse dimension of the through hole being between the first side and the second side of the through hole, 
 wherein the light source and the camera are arranged on a same side of the substrate. 
 
     
     
       13. The system of  claim 12 , further comprising a mirror configured to reflect light from the light source into the select sub-volume. 
     
     
       14. The system of  claim 13 , wherein the mirror is moveable with the substrate. 
     
     
       15. The system of  claim 13 , wherein the light source and the camera are disposed on the front side of the select sub-volume and the mirror is disposed on the back side of the select sub-volume. 
     
     
       16. The system of  claim 12 , wherein the light source is a coaxial light source.